Drill lifetime: A sample of 50 drills had a mean lifetime of 12.68 holes drilled when drilling a low- carbon steel. Assume the population standard deviation is 6.83.
a. Construct a 95% confidence interval for the mean lifetime of this type of drill.
b. The manufacturer of the drills claims that the mean lifetime is greater than 13. Does this confidence interval contradict this claim? Explain.
c. How large would the sample need to be so that a 95% confidence interval would have a margin of error of 1.0? Source: Journal of Engineering Manufacture 216: 301– 305

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